I created a custom package with 500 impressions, and have assigned two ads to it.
The campaign details says that there has been only 422 impressions, but on the same page says that the ads have received 1260 impressions.
If I add up the impressions from the Ad Manager, and the Statistics page, they add up to 1260.
So why does the campaign think its only had 422 impressions?
I've attached screenshots with the areas highlighted. I've also attached my package details so you can see how I've set it up.